Thermal control system for liquid crystal cell

ABSTRACT

A thermal control system for a liquid crystal cell is presented that may utilize a time division scheme to multiplex temperature sensing and heating functions across active thermal elements such that the cell may generally be kept at a constant temperature. A calibration process characterizes the profile of the cell and generates a polynomial regression formula that provides the voltage drive output for a temperature and cell state input. The control system stores the state of the liquid crystal cell, the regression formula, and reads the temperature of the liquid crystal cell to compute end assert the temperature compensated voltage drive.

FIELD OF INVENTION

This invention relates generally to optical liquid crystal systems. Moreparticularly, it relates to thermal control of liquid crystal cells,including liquid crystal cells integrated with optical nanostructures.

BACKGROUND OF THE INVENTION

Optical nanostructures have been the object of scientific investigationfor several years but advances in material science and imprintlithography have only recently resulted in their cost effectivemanufacturing and availability as optical components in industry.

Optical nanostructures derived with feature sizes below the wavelengthof light are known to have uniform behavior over a broad wavelength,wide acceptance angles and unique optical properties as a function ofvarying dimensions of their underlying grating features. The physicalinteraction of light with the nanostructure obeys the application ofphysics of diffraction gratings, but the scale of the structure causeschanges in the boundary effects. As a result, quantum effects influencethe classical optical effects of reflection, refraction and diffraction,resulting in nanostructure's unique optical properties. Recently, ananostructure has been created to function as a polarizing beam splitterthat performs as a perfect mirror for incident light with the lightpomarized parallel to the nanostructure and a perfect window with lightpolarized perpendicular to the nanostructure. By comparison totraditional optics, the aforementioned nanostructured polarizing beamsplitter has been demonstrated to perform with 180 degrees of beamseparation in a package size under one millimeter versus two degrees ofseparation for a 15 millimeter length of birefringent crystal.Generally, nanostructures exhibit such unique optical properties as aresult of having feature sizes in the hundreds of nanometers to tens ofnanometers, below the wavelength of incident light, eliminating allhigh-order diffractive modes and operating exclusively on zero-orderdiffraction properties.

As a result of thier low cost of manufacturing, unique opticalproperties, high performance and miniature form factor, opticalnanostructures represent a promising new technology that will have broadramifications to tomorrow's optical systems.

Realizing the performance and value of optical nanostructures istantamount to overcoming the primary challenge of integrating theseoptical structures into other optical elements. Nanostructures may beheterogeneously or monolithically integrated with other opticalelements, integrated as thin-films placed adjacent to, affixed to, orinserted into other optical components such as lasers, planar lightwavecircuits and liquid crystal devices. The challenge of integratingnanostructures with other optical elements and obtaining theextraordinary performance and scale benefits is a serious undertakinggiven that the integrated structure will carry a performance metricbased on the additive sum optical properties of the two individualstructures plus any distortion caused by the interface of thenanostructure and optical element. As a result, the performance ofintegrated structures usually do not offer the same level of highperformance provided by the nanostructure alone. There is a strong need,therefore, to increase the performance of the underlying opticalelements targeted for integration with sub wavelength optical elements.

Liquid crystal technology is known to be dynamically controlled andconfigured to enable a range of optical switching and signalconditioning applications. Formed with opposing plates of sealed glass,liquid crystal cells are considered a prospect technology andintegration target capable of supplying the active layer to ananostructure integrated therewith. Wang et. Al has recentlydemonstrated an experimental electrically tunable filter based on awaveguide resonant sub-wavelength nanostructure-grating filterincorporating a tuning mechanism in a thin liquid crystal. The deviceexperiment was functional and exhibited performance of 30 nanometertuning.

It is generally known that the performance of liquid crystal technologyis susceptible to temperature and humidity change, and that highhumidity and temperature changes cause decreased optical performance,resulting in high insertion loss and low extinction, two criticalmeasures of a cell's performance.

The speed performance and optical characteristics of the liquid crystalmedium as a function of applied electric field varies with temperature.In a liquid crystal cell relatively modest changes in temperature canresult in relatively large changes in the transmission of light, indexof refraction, and the speed of the liquid crystal state changes. FIG. 1shows the temperature influence of a liquid crystal cell index ofrefraction across voltage. FIG. 2 shows that voltage for a selectedtransmission of light in one temperature range will provide a differenttransmission of light at different temperatures. FIG. 3 shows therelationship between temperature and the amount of time it takes aliquid crystal cell to change states, which decreases with increasedtemperature. FIG. 3 also shows that switching times of liquid crystalcells are sensitive to cell gap thickness. The series represented in thefigure are two cells each having different gap sizes. More specifically,the faster switching cell has a cell gap 0.4 micron larger than theslower cell. Clearly, size and the effect of the change in opticalproperties are factors in controlling the optical performance in thevarious states of the liquid crystal cell across temperature.

In order to ensure that the temperature of the liquid crystal medium canprovide stable operation and within a practical response time, prior artliquid crystal cells are known to utilize active thermal managementsystems based on independent temperature sensor and heater elements.JACKSON et al. relies on a resistive heating element that can beenergized to heat the liquid crystal cell whenever the temperature ofthe cell drops below a predetermined temperature trip point. JACKSONdoes not accommodate feedback to the voltage control of the cell andfails to handle ambient temperature increases above the trip point.McCartney et al. provides a more complete solution that incorporates theoutput of the temperature sensor into a temperature feedback loop toadjust voltage in response to temperature change. In this design, atwo-dimensional lookup table provides the output voltage for anytemperature and pixel attribute combination. McCartney's design,however, does not scale to high resolution optical systems withoutincreasing the size of the lookup table.

In general, the prior art liquid crystal thermal management systems relyon use of individual discreet devices for heating and sensing the liquidcrystal cell. These devices are generally affixed to the outside glassof the cell at disparate locations so they are generally incapable offunctioning uniformly across the cell. In addition, because thesedevices are usually affixed to the outside glass, all heating andsensing functions directed to the liquid crystal molecules on the insideof the glass must be translated through the glass medium. This canresult in hysterises and other effects that distort the effectiveness ofclosed loop temperature sensing and heating systems. Finally, prior-artliquid crystal cell heaters and temperature sensors are typicallyattached to the cell using epoxy resins, and epoxy resins are generallyknown to absorb moisture in high temperatures and high humididtyconditions, which leads to degradation or inconsistancy in cellperformance.

The performance of liquid crystal cells are generally very sensitive tomoisture and humidity. Prior art liquid crystal seals are known toprovide varying levels of protection of liquid crystal cells frommoisture and humidity. The prior art designs generally seal and spacethe cell with glass beads, frit and organic polymers such as epoxyresin. Sealing materials are generally disposed, in the form of gaskets,about the periphery of the cell. The advantage of a seal of glass fritis known to be that such seal is practically impervious to gas andvapors, but this approach requires formation by high temperatureprocessing, and high temperature processing tends to distort thesubstrate and render difficult control uniformity of the distancebetween the inner surface of the parallel substrates. This gap(containing the liquid crystal material) must be maintained with a highdegree of uniformity to achieve precise operation of a liquid crystalcell. Accurately controlling the liquid crystal cell gap is keystone toenabling high performance nanostructured liquid crystal optical systemsof the present invention.

In producing an effective glass frit seal, the frit is generally appliedto a surface of one of the substrates as a paste of glass powderparticles dispersed in a liquid vehicle. The substrate is subsequentlyheated over a programmed temperature regime wherein, at lowertemperatures, the solvent is evaporated and the binder is burned off,and hence in the higher temperature portions of the regime, the glasspowder itself melts and coalesces to form a strongly adhesive bond tothe glass substrate. Subsequently, the second glass substrate ispositioned over the coalesced frit and the entire assembly is againsubjected to a programmed temperature regime during which thetemperature is raised within a few tens of degrees of the glazingtemperature of the glass frit. At this relatively high temperature, theglass frit wets the second substrate to acquire satisfactory adhesionthereto. It is known that this second heating cycle tends to soften thesubstrates and cause warpage thereof, with the result that cells,particularly those of larger surface area, sealed by this glass fritmethod tend to have a very low percentage of acceptable manufacture.

It is generally known that warpage during fabrication can be preventedby the alternate use of organic polymer sealants, such as epoxy resinsand the like, which can be processed at much lower temperatures. Polymersealants may be screen printed from a solution or dispersion of thepolymer in a solvent, or a polymer sheet can be cut into the shape of agasket which is sandwiched between the substrates to be sealed, and thesandwich is subsequently heated to effect such seal. It is also known tointroduce the polymer along the edges of an assembly of two substrateswhich are kept otherwise separated by interior spacers. However, suchorganic polymer sealants have a relatively high permeability to watervapor. Under high temperature and humidity conditions, water vaporpermeates into the seal causing the expansion of the seal and a shapechange in the liquid crystal cavity that results in a change in theknown performance of the liquid crystal cell.

FEATURES OF THE INVENTION

The present invention contain several features that may be configuredindependently or in combination with other features of the presentinvention, depending on the application and operating configurations.The delineation of such features is not meant to limit the scope of theinvention but merely to outline certain specific features as they relateto the present invention.

It is a feature of the present invention to provide a liquid crystalcell that may be formed of glass etched with sub wavelength opticalfeatures.

It is a feature of the present invention to provide a liquid crystalcell that may be fabricated with an integrated sub wavelength opticalnanostructure.

It is a feature of the present invention to provide a liquid crystalcell that may be fabricated with an integrated optical element.

It is a feature of the present invention to provide a liquid crystalcell that may be fabricated with an integrated polarizer, beam splitter,polarization beam splitter, waveguide, thin film, filter, mirror,photodetector, isolator, grating, subwavelength grating, combiner,waveplate, nanostructure, or some combination thereof.

It is a feature of the present invention to provide a liquid crystalcell platform that can be configured in various applications, includingbut not limited to telecommunications applications in optical switching,variable optical attenuation, tunable filters and wavelength selectionapplications.

It is a feature of the present invention to provide a liquid crystalcell that may be constructed from materials substantially impervious tomoisture.

It is a feature of the present invention to provide a liquid crystalcell that may contain a heater and temperature sensor integrated thereinas single physical element and to provide for accurate control ofheating and temperature sensing.

It is a feature of the present invention to provide a novel method ofoperating a liquid crystal cell across a range of temperature withoutthe need for lookup tables otherwise used to compensate for real timetemperature changes.

It is a feature of the present invention to provide a liquid crystalcell that passes the strict telecommunications guidelines as outlined inTelcordia GR1221 without the need for hermetic housing.

It is a feature of the present invention to provide an optically flatliquid crystal cell and not otherwise prone to warpage duringfabrication process.

It is a feature of the present invention to provide an optically flatliquid crystal cell and not otherwise prone to warpage when exposed tovarious thermal and humidity atmospheres.

It is a feature of the present invention to provide a liquid crystalcell whose thickness may be controlled at nanometer resolution.

It is a feature of the present invention to provide a novel method forfabricating a liquid crystal cell having some or all of the featuresincluded therein.

It is a feature of the present invention to provide a novel method foraligning two substrates, including but not limited to those substratesof the present invention.

It is a feature of the present invention to provide a platform that maybe used in transmissive or reflective liquid crystal cells.

It is a feature of the present invention to provide a platform that maybe configured into an array of liquid crystal cells.

SUMMARY OF THE INVENTION

The disadvantages associated with the prior art may be overcome by aliquid crystal cell and fabrication method directed to a deposited metalgasket moisture barrier bonding two opposing plates of glass each havinga spacer layer to accurately control cell gap thickness. The liquidcrystal cell may include an integrated thermal sensor and heaterdeposition layer sandwiched between or deposited on at least one or bothopposing plates of glass.

The disadvantages associated with the prior art may further be overcomewith a liquid crystal cell control system utilizing a time divisionscheme that multiplexes temperature sensing and heating functions acrossan integrated active thermal element such that the cell may generally bekept at a constant temperature. A calibration process characterizes theprofile of the cell and generates a polynomial regression formula thatprovides the voltage drive output for a temperature and cell stateinput. The control system stores the state of the liquid crystal cell,the regression formula, and reads the temperature of the liquid crystalcell to compute and assert the temperature compensated voltage drive.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows one example liquid crystal cell's temperature dependenceand change in index of refraction.

FIG. 2 shows one example liquid crystal cell temperature dependence ofthe attenuation function.

FIG. 3 shows how liquid crystal cell shutter switching speeds change asa function of temperature and cell gap thickness.

FIGS. 4A-4D shows various liquid crystal cell embodiments of the presentinvention.

FIG. 5 shows one process flow for fabricating the liquid crystal cellsof the present invention.

FIGS. 6A and 6B show example indium tin oxide (ITO) electrode formingmasks of the present invention.

FIGS. 7A and 7B show example integrated active thermal element formingmasks of the present invention.

FIGS. 8A and 8B show example spacer element forming masks of the presentinvention

FIGS. 9A and 9B show example masks for defining a metal gasket elementlayer of the present invention.

FIG. 10A shows a top view example integrated perspective showing therelationship between various layers of the present invention.

FIG. 10B is an isometric view showing a liquid crystal cell at thetermination of the fabrication process.

FIG. 11 shows the liquid cyrstal thermal calibration and feedback loopmethod flows.

FIG. 12 shows a block system diagram for the electronic control andthermal management system of the present invention.

DETAILED DESCRIPTION

Throughout this application, like reference numbers as used to refer tolike elements. For instance, the two substrates used to form the liquidcrystal cell of the present invention are referred to throughout thisapplications as 110A and 110B. Those supporting elements and features ofthe invention that are distributed on each substrate and later combinedmay be referred to under their index reference for a particularsubstrate 'A, 'B or for simplicity sake, under the shared reference '.

A first embodiment of the present invention is presented in FIG. 4A,which shows (not to scale) a liquid crystal cell platform 100 having afirst substrate 110A in opposition to a second substrate 110B. In thisembodiment, the first substrate may contain an inner surface having atransparent conductive electrode layer 104A, liquid crystal alignmentlayer 109A, a metal gasket element layer 106A and spacer element layer107A. The second substrate 110B contains an inner surface having an atransparent conductive electrode layer 104B, a liquid crystal alignmentlayer 104B, metal gasket element layer 106B and spacer element layer107B.

A second embodiment of the present invention includes an integratedoptical element 111 and is presented in FIG. 4B, which shows (not toscale) a liquid crystal cell platform 100 having a first glass substrate110A in opposition to a second glass substrate 110B wherein the firstsubstrate contains an integrated optical element 111 on one side of thesubstrate, a transparent conductive electrode layer 104A, a liquidcrystal alignment layer 109A, metal gasket element layer 106A and spacerelement layer 107A on the opposing side, and, the second substrate 110Bcontaining a transparent conductive electrode layer 104B, a liquidcrystal alignment layer 109B, metal gasket element layer 106B and aspacer element layer 107B.

A third embodiment of the present invention is presented in FIG. 4C,which shows a liquid crystal cell platform 100 having a first glasssubstrate 110A in opposition to a second glass substrate 110B whereinthe first substrate contains an integrated optical element 111, atransparent conductive electrode layer 104A, a liquid crystal alignmentlayer 109A, a metal gasket element layer 106A, a spacer element layer107A and an integrated heater/temperature sensor element layer 108A. Inthis embodiment, the second substrate 110B contains a transparentconductive electrode layer 104B, a liquid crystal alignment layer 104B,metal gasket element layer 106B, a spacer element layer 107B, and anintegrated active thermal element, heater/temperature sensor layer 108B.

A fourth embodiment of the present invention is presented in FIG. 4D,which shows a liquid crystal cell platform 100 having a first glasssubstrate 110A in opposition to a second glass substrate 110B whereinthe first substrate contains an integrated optical element 111, atransparent conductive electrode layer 104A, a liquid crystal alignmentlayer 109A, a metal gasket element layer 106A, and a spacer elementlayer 107A. In this embodiment, the second substrate 110B contains anintegrated optical element 112, transparent conductive electrode layer104B, a liquid crystal alignment layer 104B, metal gasket element layer106B, and a spacer element layer 107B. In this embodiment, theintegrated optical elements 111 and 112 may provide the same ordifferent functionality, depending on the application. For example, in afree space variable optical attenuator application, a transmissive cell100 might be configured with two polarizers transmitting perpendicularstates of light, 111, 112, respectively. In an optical switchingapplication, a reflective cell 100 may include an optical element 111functioning as a polarization beam splitter and combiner, and opticalelement 112 functioning as a mirror.

With respect to all embodiments, it is generally preferable thatsubstrate 110 be comprised of glass but other substrate materials,including silicon, polymers, etc., may be suitable depending on theapplication.

FIG. 5 shows one example fabrication process to create the liquidcrystal cell platform 100. Various optional steps may be omitteddepending on the embodiment of configured features.

With respect to FIG. 5, optional step one involves integrating anoptical element into at least one substrate. The optical element mayfunction as a polarizer, beam splitter, filter, thin film, polarizationbeam splitter, waveguide, waveplate, combiner, mirror, partiallytranparent mirror, isolator, detector, grating, subwavelength grating,nanostructure, or some combination thereof and including those opticalfunctions presently known in the art. Preferably, the optical element isa nanostructured grating feature, such as those described by NanoOptoCorporation of New Jersey. The grating feature may be applied to orintegrated onto substrate 110A or 110B, or onto both substratesdepending on the application. With respect to process step 201, a glasssubstrate is etched using nanoimprint lithography or similar methodsknown in the field based on impressing a reference mask into photoresist to create surface relief patterns on the substrate where thesurface relief photo resist pattern is etched to form grating featuresin the nanometer range. Alternately, the optical element may be suppliedas a thin film and bonded to the target substrate by way of epoxy orother methods described herein or otherwise generally known. The opticalelement may also be deposited directly on the inner or outer surface ofeither substrate, or both. Finally, the optical element itself may beintegrated into the substrate by way of choice of substrate material.For example, the substrate 110A, 110B or both substrates may be made ofPolarcor, a polarization beam splitter glass made by Corning, Inc.

Step two involves adding the appropriate ITO patterns to the first andsecond glass substrates to form the liquid crystal electrodes. Withrespect to process flow 202 of FIG. 5, a standard PECVD process may beused to apply thin film of ITO approximately 100 angstroms thick. FIGS.6A and 6B show example ITO masks that may be used to pattern substrates110A and 110B, respectively.

Step three involves adding polyimide alignment layer to the first andsecond glass substrates. With respect to process flow 203 of FIG. 5,standard spin coating stepped processes may be used at room temperatureto create a layer of polyimide approximately 600 angstroms thick on eachsubstrate.

Step four involves patterning the polyimide layer. With respect toprocess 204, photo resist may first be applied to the substrates andmasked using traditional photolithography techniques or laser etchingmay be used to pattern the substrates. Wet or dry etching performedthereafter may result in a pattern of polyimide.

Step five involves anchoring the liquid crystal alignment layers. Withrespect to process step 205, one traditional method is to rub thepolyimide of each substrate to form the alignment layers. In a twistednematic configuration, the rubbing direction of the first substrate maybe orthogonal to the rubbing direction of the second substrate. In anelectronically conductive birefringence (ECB) configuration, the rubbingdirection of the first substrate may be parallel to the rubbingdirection of the second substrate. Various anchoring schemes may bedefine rub angles other than 0 or 90 degrees. An alternate method offorming the alignment layers is to employ an imprint lithographytechnique where a reference mask is pressed onto a deposited photoresist layer to create surface relief patterns in the photo resist whichis subsequently form etched to high precision alignment grooves withnanoscale tolerance.

Optional step six involves creating the active thermal element,integrated heater and temperature sensor. FIGS. 7A and 7B show examplemasks that may be use with respect to process step 206 of FIG. 5, inwhich a seed adhesion layer of chrome is first deposited approximately200 angstroms thick onto the substrates, followed by a PECVD depositionthin film platinum resistor layer approximately 2000 angstroms thick andforming the upper and lower portions of the integratedheater/temperature sensor. The upper and lower portions of theintegrated device, applied to substrates 110A and 110B, may be separatedby an air gap approximately 9.6 microns and interconnected by VIASformed from a metal deposition step that will be described in succeedingstep eight. Again, it need be stated that gap thickness is delineatedfor example purposes and will change depending on the desiredapplication. It should be stated that, depending on the configuration,the platinum thin film resistor may be patterned in various shapes,including but not limited to arched, curved, circular, zigzag, strippedas well as the serpentine pattern of FIGS. 7A and 7B. Given theresistivity of the thin film platinum, approximately 10.6E-8 ohm meters,the example shown yields approximately 100 ohms resistance at roomtemperature.

Step seven involves creating the spacer element 107. Spacer element 107controls the gap thickness of the liquid crystal cell. While it is notnecessary to equally distribute the spacer element equally on eachsubstrate, it is preferred that one half of the desire gap thickness ofthe completed cell shall define the thickness of the spacer element 107as deposited on each substrate. The combined cell 100 gap thickness maytherefore be formed with a tolerance based on deposition process.Silicon dioxide is the preferred material for creating the spacerelement, however other materials such as aluminum oxide, siliconnitride, silicon monoxide and other materials compatible with thin filmdeposition processes that do not substantially compress may also be usedas an alternative to the silicon dioxide provided they are compatiblewith the selected liquid crystal substrate material. FIGS. 8A and 8Bshow an example mask that may be used to perform the process step 207 ofFIG. 5, where a patterned layer of 5 microns thick of silicon dioxide isdeposited onto each substrate.

Step eight involves creating the metal gasket element 106. Metal gasketelement 108 may be made from a variety of metals, including but notlimited to, indium, gold, nickel, tin, chromium, platinum, tungsten,silver, bismuth, germanium and lead. However it is preferable to useindium because of its pliability and relatively low melting temperature.FIGS. 9A and 9B show example masks that may be used to perform processstep 208 of FIG. 5, where, for the continuing example purpose, a layerapproximately 7 to 9 microns thick of indium may equally be deposited oneach substrate. It is generally preferable that metal gasket layer ofthis process step is deposited thicker than the spacer element of theprevious step due to seepage that occurs during the additionalprocessing steps. Metal gasket masks, such as those shown in FIGS. 9Aand 9B, may be configured to form referential VIAS 300 that enableelectrical interconnection between features deposited on eithersubstrate 110A or 110B. VIAS 300 may also be formed to simplify routingexternal contact pads to the temperature sensor and heating element. Forexample the VIAS 300 of the present example are positioned to overlapthe heater/temperature sensor platinum layer defined in step six. Theyare also positioned to overlap the ITO layer so as to define contactpads to drive the two electrodes of the liquid crystal cell.

Step 9 involves aligning and pressing wafers 110A together with 110B. Itis known that visual alignment reference marks may be etched into theunderlying wafer, or that a physical feature of the glass sheet such asan edge or alignment hole may be used to perform wafer alignment.However, a high yield method of accurately aligning the relativeposition of the two glass substrates without the need for expensive highprecision alignment equipment is hereby presented, in whichcomplimentary interlocking geometric features deposited on eachsubstrate, mate with each other to prevent relative movement of theglass sheets during the bonding and pressing process. Such interlockingfeatures mitigate any non uniformity in the bonding process and giventhat the typical gap between two glass sheets of a liquid crystal cellis less than 20 micrometers, thin film deposition or screening processescan be used to create precisely controlled and repeatable geometricfeatures. With respect to process step 209 of FIG. 5, the substrates110A and 110B may be brought together, aligned under pressure at roomtemperature to form a chemical bond metal gasket at the gap distancedefined by the sandwich spacer elements formed from both substrates.

Step 10 involves dicing of the wafers. Process step 210 of FIG. 5 may beperformed using a dicing saw or via etching techniques.

Step 11 involves removal of a portion of protective glass on the liquidcrystal cell. FIG. 10A shows a top perspective of the various layersthat combine through the substrates when interposed thereupon each otherin a fully configured embodiment of the present invention. With respectto process 211 of FIG. 5, the substrate 110B is scored using a diamonddicing saw to cut a trench approximately 90% through the thickness ofthe substrate and forming the break off line 119 of FIG. 10A. A portionof the substrate 110B is broken off along the break off line 119 todefine an access surface 113 of FIG. 10B that provides access to theunderlying liquid crystal electrode contact pads 500 and 500′, theunderlying liquid crystal heater/temperature sensor element electricalcontact pads 502 and 502′, as well as to the liquid crystal fill port115.

Step 12 involves filling the liquid crystal device with a liquid crystalmolecules, process 212 of FIG. 5. This step may be performed usingtraditional methods of filling a liquid crystal cell, whereby the cellis placed in a vacuum, a droplet size of liquid crystal material isplaced at the fill port 115, and with the release of the vacuum,equilibrium pressure forces the liquid crystal material into the fillport 115 and the fill port is plugged. Several techniques to cap thefill port, including UV curable epoxy which may be used to close thefill port.

Electronic Control System

A block diagram of components directed to a liquid crystal cell systemand its host controller are included in FIG. 11 along with the liquidcrystal thermal management and voltage controller subsystems of thepresent invention, now described in further detail.

In one example configuration, host computer 400 may be configured tocommunicate with microcontroller 402 over a full duplex data interfaceand enabling the host computer to engage functions, send commands andretrieve data from microcontroller 402. Microcontroller may beconfigured to store software control routines. The software controlroutines may function to adjust voltage drive provided to the liquidcrystal cell in response to temperature fluctuations.

The microcontroller may utilize a time division multiplexing scheme thatmultiplexes temperature sensing and heating functions in the integratedsensor/heater device such that the cell may generally be kept at aconstant temperature. A calibration process characterizes the profile ofthe cell and generates a polynomial regression formula that provides theoptimal voltage drive output for given temperature and cell stateinputs. The microcontroller 402 stores the state of the liquid crystalcell, the regression formula, and reads the temperature of the liquidcrystal cell to compute and assert the temperature compensated voltagedrive.

FIG. 11 shows a calibration process that may be used to perform themethod of the present invention in which a liquid crystal cell thermaloperating characteristic profile is translated into deterministiccoefficients assembled into a stored regression formula used to adjustthe voltage drive to the cell in response to temperature and cell state.

The first step to determine the coefficient values in the cell'stemperature and voltage compensation profile, is to profile the liquidcrystal cell drive characteristics across a range of temperatures. Theprofile process step 601 may examine a light source passing through thecell and its attenuation at a given voltage and temperature combination.An operational liquid crystal cell is placed in a thermal chamberprogrammed to change operating temperature across the desiredtemperature range at a given interval. At every temperature changeinterval, a range of voltages are provided to the liquid crystal cellwhile a performance characteristic, such as attenuation, is measured.Voltage is scanned until reference attenuation levels are achieved, atwhich point the voltage, attenuation and temperature levels are storedas a grid reference in a cell profile definition table. The performanceof the liquid crystal cell is recorded at grid point attenuation andtemperature levels, resulting in a multi dimensional lookup tablewhereby any temperature and voltage input provides an attenuation leveloutput. This table may be represented as a three dimensional surface.

Step two requires processing the lookup table to smooth the voltageprofile over temperature at the given attenuation levels as recorded inthe previous step. A statistical program capable of performingregression analysis, such as Mathematica® may be used to perform thisprocess step 602. The regression software is provided with the look uptable generated in step one, and performs a fourth order regressioncurve fitting process that generates for each attenuation level, theappropriate coefficients a,b,c,d, and e representing a voltage versustemperature profile of the cell at each attenuation level, representedby the following formula,v=a+bT+cT ² +dT ³ +eT ⁴ v ₁ =a ₁ +b ₁ T+c ₁ T ² +d ₁ T ³ +e ₁ T ⁴v ₂ =a ₂ +b ₂ T+c ₂ T ² +d ₂ T ³ +e ₂ T ⁴***v _(n) =a _(n) +b _(n) T+c _(n) T ² +d _(n) T ³ +e _(n) T ⁴where V=voltage, T=liquid crystal cell temperature, a,b,c,d,e=curve fitcoefficients, and n=attenuation level.

Given that smooth curves result from the prior step that define theoptimal voltage drive level for a given temperature at the recorded gridattenuation level, step three results in smooth curve regressions fitacross orthogonal axis of the three dimensional surface, whereby thesmooth curves are fit over the coarse attenuation grid recorded in step1. In this process step 603, the five coefficients of the previous stepare each solved by a second order regression. Specifically, Mathematica®or any suitable program is used to solve for the three coefficients thatfit the profile of each of the five coefficients a,b,c,d and e acrossall of the orders of the regressionv_(n)=a_(n)+b_(n)T+c_(n)T²+d_(n)T³+e_(n)T⁴. So, a smooth surface profiledefines the optimum voltage compensation level given an inputattenuation state and temperature by the following formulav=a+bT+cT ² +dT ³ +eT ⁴, where, a=(X+Yθ+Zθ ²)b=(X ₁+Y₁θ+Z₁θ²)c=(X ₂+Y₂θ+Z₂θ²)d=(X ₃+Y₃θ+Z₃θ²)e=(X ₄+Y₄θ+Z₄θ²)Theta=liquid crystal attenuation level

-   X,Y,Z=solution to zero order coefficient-   X₁,Y₁,Z₁=solutions to first order coefficient-   X₂,Y₂,Z₂=solutions to second order coefficient-   X₃,Y₃,Z₃=solutions to third order coefficient-   X₄,Y₄,Z₄=solutions to fourth order coefficient

The fifteen coefficient solutions (Xn,Yn,Zn) where n=0 to 4, may begenerated by Mathematica, using the Fit(data, {1,x,x^2, . . . ,x^n},x)function or other suitable software packages capable of performing curvefitting regression.

Step four is the final step in the calibration process of FIG. 11,process 606, and results in storing the coefficients in the liquidcrystal control system which is now described.

The coefficients that profile the liquid crystal characteristics may bestored in microcontroller 402 memory (FIG. 12) by flashing the memory ofthe microcontroller with the appropriate 15 coefficient values.

The thermal compensation system of the present invention operates byreading the temperature of the liquid crystal cell and adjusting thevoltage drive of the cell based on the cell state. The cell state maytypically be OFF, ON or operate in a variable mode. The cell state maybe stored in the microcontroller 402 and also be configured via the hostcomputer 400.

Microcontroller may be a PIC microchip having an internal analog digitalconverter and operating with a 10 Mhz crystal oscillator 404 clock. Themicrocontroller may be connected to a digital analog converter (DAC)configured to provide an output voltage level in response to aconfiguration pulse stream from the microcontroller over a serialinterface. The output of the DAC connects to the input of an analogswitch 414 which is clocked by a port pin of the microcontroller atapproximately 1.2 khz. DATA passed to the DAC defines the amplitude ofan AM transmission over a 1.2 khz carrier that produces a differentialvoltage drive to the liquid crystal cell electrodes 500 and 500′ (FIG.10B).

A temperature sensor reading may be provided by the internal integratedheater/temperature sensor from an external device. One of theheater/temperature sensor electrodes 502 or 502′ of the liquid crystalcell 100 may be grounded while the other may connect to switch 407.Switch 407 may selectively engage the integrated heater/temperaturesensor element 108 in a sense or heat mode. More specifically, switch407 may be configured ON to connect the ungrounded heater/temperatureelectrode through instrumentation amplifier 406 to an ADC coupled to themicrocontroller which reads the temperature on the liquid crystal cell,or it may be configured OFF so that power amplifier FET 410, which maybe controlled by a pulse train from microcontroller 402 and applies avoltage potential to operate the device 108 as a heater.

In a temperature sense feedback closed loop operation, which shallhereby be referred to as the loop embraced by process steps 607 through609 of FIG. 11, the microcontroller reads the temperature of the liquidcrystal cell and calculates the voltage drive based on the sensedTemperature, T, and the current state of the liquid crystal cell, Theta.The fifteen coefficients are plugged back into the fourth orderregression formula to establish a smooth surface profile delineating anoptimal voltage to supply to liquid crystal cell for a given temperatureand cell attenuation level:v=(X+Yθ+Zθ ²)+(X ₁ +Y ₁ θ+Z ₁θ²)T+(X ₂ +Y ₂ θ+Z ₂θ²)T ²+(X ₃ +Y ₃ θ+Z₃θ²)T ³+(X ₄ +Y ₄ θ+Z ₄θ²)T ⁴

The new voltage value V is computed and transmitted to the DAC 412 whichsupplies the appropriate amplitude DC voltage into the clocked analogswitch 414 to produce the temperature compensated AM voltage drive tothe liquid crystal cell.

The liquid crystal cell may also be maintained about a referencetemperature. Process step 609 with respect to FIG. 11 involves theapplication of heat to maintain the temperature of the liquid crystalcell about a reference temperature. The reference temperature may beabove the ambient room temperature or above the temperature of anycarrier device that may be coupled to the LC cell. The selection of areference temperature above the ambient temperature will result in thetendency of the LC cell to cool to meet the ambient temperature afterthe application of a heat burst. A counter thermal bias is thereforegenerated to support temperature stability about the referencetemperature.

Microcontroller memory may store the reference temperature, the value ofthe current temperature, historical temperatures, and, historical levelsof heat applied to the LC cell. The value of the sensed temperature T atevery instance may be compared against the reference temperature todetermine the amount of heat to apply to the liquid crystal cell. An 8bit analog digital converter will provide approximately ⅓ or a degree oftemperature sensing resolution over the desired temperature range, sothe example system may provide for temperature stability about areference temperature to within ⅓ degree Celsius. At every instance ofprocess step 609, a threshold detector routine stored in microcontrollerROM may trigger a control function if the sensed temperature of theliquid crystal cell falls below the desired operating referencetemperature. The control function may determine how much heat to applyto the liquid crystal cell. The control function may utilize errorminimizing routines that track the change in temperature across multipleinstances of process step 609. The error correcting routines may storethe previous temperature reading T0 along with the previous amount ofheat applied to the liquid crystal cell H0. The temperature reading andevery succeeding temperature reading T1 may be compared against T0 todetermine the amount of temperature change resulting from the previousheating of the liquid crystal cell. Heat may be applied to the liquidcrystal cell by way of the FET power driver as described above. Theheater may be triggered at a fixed or variable duty cycle and controlledusing frequency or amplitude modulation.

Although the present invention has been fully described by way ofdescription and accompanying drawings, it is to be noted that variouschanges and modifications will be apparent to those skilled in the art.For example, various patterns may be used to form the spacer element,metal gasket and integrated heater/temperature sensor elements of thebasic cell platform. Use of external temperature sensors and heaters inpart or whole may be applied using the temperature compensation methodsand regression of the present invention. The metal gasket may bemodulated to provide heating function in addition to its function as amoisture barrier support membrane. Epoxy gaskets may be used incombination with metal gasket elements in part or whole, and the metalgasket elements may comprise a single solder cap. Anchoring and aligningthe liquid crystal material in a cell may also be performed using photoalignment material, Staralign by Vantio of Switzerland or or other knownalignment methods, including laser etching. Anchoring the liqiud crystalmaterial in the cell (described hereunder as step five) may beperformmed before patterning of the polyimide (describedc hereunder asstep four). The process steps for the closed loop temperature feedbackmay also be rearranged such that the heating process is performed priorto applying the voltage drive. The order of fitting voltage with eachdimension of the three dimensional surface is reversable and other threedimensional surface fitting algorithms may be used, including but notlimited to those that describe a surface with one dimension fitting afourth degree polynomial and the other dimension fitting a second degreepolynomial. Amplitude or frequency modulation may be used to drive theliquid crystal cell. The fourth embodiment of this invention may beconfigured with the integrated temperature sensor/heating element of thethird embodiment of the present invention. The liquid crystal cell maynot be limited to a single pixel. The liquid crystal cell may becomprised of multiple pixels. Arrays of liquid crystal cells may beformed, including arrays of cells having one or more pixels. Therefore,it is to be noted that various changes and modifications from thoseabstractions defined herein, unless otherwise stated or departing fromthe scope of the present invention, should be construed as beingincluded therein and captured hereunder with respect to the claims.

1. A method of operating a liquid crystal cell such that the performanceof the liquid crystal cell is not effected by temperature, comprisingthe steps of: A) performing a calibration process that results in amulti dimensional lookup table that produces an output voltage value forany given temperature and attenuation level, B) consolidating the lookuptable from step a into a calibration formula, C) providing thetemperature of the liquid crystal cell, D) providing the required stateof the liquid crystal cell, E) computing an output voltage value usingthe formula of step B given inputs values derived from the steps C andD.
 2. The method of claim 1, further including after step E, a step Fwhereby the liquid crystal cell temperature is maintained about areference temperature.
 3. The method of claim 2, further including afterstep F, step G whereby steps C through G are repeated.
 4. The method ofclaim 1, wherein the formula that defines the optimal voltage to applyto the liquid crystal cell is provided by:v=(X+Yθ+Zθ ²)+(X ₁ +Y ₁ θ+Z ₁θ)T+(X ₂ +Y ₂ θ+Z ₂θ²)T ²+(X ₃ +Y ₃ θ+Z₃θ²)T ³+(X ₄ +Y ₄ θ+Z ₄θ²)T ⁴ Wherein T may equal temperature of theliquid crystal cell, theta the current cell state or attenuation level,and Xn, Yn, and Zn coefficients derived by the output of five secondorder regressions.
 5. The method of claim 4, wherein the fifteencoefficients generated in step B are stored in control system memory.